Neptune-like planet orbiting a dead white dwarf star discovered

In a first, a giant Neptune-like planet orbiting a dead white dwarf star has been discovered in the form of a disc of gas formed from its evaporating atmosphere. The planet orbits a star a quarter of its size about once every ten days, leaving a comet-like tail of gas comprised of hydrogen, oxygen, and sulphur in its wake. The discovery by astronomers from the University of Warwick's Department of Physics and the Millennium Nucleus for Planet Formation (NPF) at the University of Valparaíso was published on December 4 in the journal Nature.

The discovery suggests that there could be many more planets around such stars waiting to be found. Until now, there has never been evidence of a planet that has survived a star's transition to a white dwarf. The star WDJ0914+1914 was identified in a survey of 10,000 white dwarfs observed by the Sloan Digital Sky Survey. Scientists at Warwick analysed subtle variations in the light emitted from the system to identify the elements present around the star.

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A white dwarf with disc 10x size of Sun

Lead author Dr Boris Gaensicke, from the University of Warwick said, "At first, we thought that this was a binary star with an accretion disc formed from mass flowing between the two stars. However, our observations show that it is a single white dwarf with a disc around it roughly ten times the size of our sun, made solely of hydrogen, oxygen, and sulphur. Such a system has never been seen before, and it was immediately clear to me that this was a unique star."

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A glimpse into our future

Through a set of calculations, Dr Matthias Schreiber from the University of Valparaíso showed that the 28,000 degree Celsius hot white dwarf is slowly evaporating the planet by bombarding it with high energy photons. The white dwarf is sucking the planet's lost mass into a gas disc around the star at a rate of over 3,000 tons per second. Schreiber says, "In a sense, WDJ0914+1914 is providing us with a glimpse into the very distant future of our own solar system." This white dwarf was once a star similar to the Sun but eventually ran out of fuel, swelled up into a red giant, a few 100 times the size of the sun. During that phase of its life, the star will have lost about half of its mass and what was left has shrunk dramatically ending up size of the Earth – the white dwarf is essentially the burnt-out core of the former star.

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Once our own Sun runs out of fuel in about 4.5 billion years, it will shed its outer layers, destroying Mercury, Venus, and probably the Earth, and eventually expose the burnt-out core – the white dwarf. In a companion paper led by Dr Schreiber and Dr Gaensicke and published in Astrophysical Journal Letters, they detail how this will radiate enough high energy photons to evaporate Jupiter, Saturn, Uranus, and Neptune. Just as on WDJ0914+1914, some of that atmospheric gas will end up on the white dwarf left behind by the Sun, and will be observable for future generations of alien astronomers.

The astronomers argue that this planetary evaporation and subsequent accretion by young white dwarfs is probably a relatively common process and that it might open a new window towards studying the chemical composition of the atmospheres of extrasolar gas giant planets.

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(With inputs from agencies)